Allow disable flush on background

[jakecronin]

Mixpanel is causing occasional terminations 30 seconds after our app enters the background due to long-running background tasks.

[BackgroundTask] Background Task 113 ("Called by Siro, from $s8Mixpanel0A8InstanceC29applicationDidEnterBackground33_0CB45E1C8B4A4433813A640264188496LLyy10Foundation12NotificationVF"), was created over 30 seconds ago. In applications running in the background, this creates a risk of termination. Remember to call UIApplication.endBackgroundTask(_:) for your task in a timely manner to avoid this.

Adding the ability to disable flush on entering background allows us to sidestep this issue.

Future work should be done to alow Mixpanel to fail gracefully if a background task is taking too long rather than causing app to terminate.

[zihejia]

thanks @jakecronin , could you fix the android test? In the meantime, we have this PR from iOS which should be able to fix the root cause as well. We will do a release tomorrow on the iOS side.

[jakecronin]

No problem @zihejia. Looks like I falsely assumed the existence of an android method.

I just pulled out the android support and added a platform check to make sure this is iOS only.

[zihejia]

thanks @jakecronin